1. Field of the Invention
This invention relates to a semiconductor light detecting device for used in receivers, etc. of optical communication systems.
2. Related Background Art
In a semiconductor light detecting device for use in the optical communication using optical fibers, as exemplified in FIG. 1 (a first prior art), conventionally a photodiode chip 1 as the light detecting means is mounted on a header 2, and a lead wire 3 is bonded to the photodiode chip 1, whereby the signal light emitted from the end surface of an optical fiber 6 inserted by means of a ferrule 7 in a housing 8 is taken out to be converted into an electric signal.
A cap 4 airtightly seals the photodiode chip 1 and has a light transmitting window 5 which passes the signal light.
A condenser lens 9 is used as means for effectively applying the signal light emitted from the end surface of the optical fiber 6. to the photodiode chip 1. The device of FIG. 1 is called coaxial type.
On the other hand, a semiconductor light detecting device (a second prior art) shown in FIG. 2 is called butterfly type. The device of this type has a housing 8 of rectangular section. This type has the same function as the first prior art of FIG. 1, but it is possible to disposed an IC (integrated circuit) on a header 2 secured inside a housing 8 to amplify to some extent a photocurrent (photoelectric current signal) generated in the pn junction of the photodiode chip 1 as the light detecting means and take out the amplified photocurrent.
The photodiode chip 1 light detecting means comprises, as shown in FIG. 3, an epitaxial layer (semiconductive crystal layer) 12 grown on a semiconductor substrate 11, and a diffusion region (first region) 13 formed on a surface of the epitaxial layer 12 and having a polarity opposite to that of the epitaxial layer 12 by diffusing a metal element.
Since the interface between the epitaxial layer 12 and the diffusion region 13 has a pn junction, a generated photoelectric current is taken out as an electric signal through electrodes 14, 17.
FIGS. 4 and 5 respectively show a top view and an X--X sectional view of a structure of the photodiode chip 1 described above (FIG. 3). As shown, in the photodiode chip 1, a semiconductive crystal layer 12 of a first conductivity type including a light absorption layer is laminated on a surface of a semiconductor substrate 11 of a first conductivity type having an electrode 17 of the first conductivity type formed on an underside there, and impurities are selectively diffused into the semiconductive crystal layer 12 to form a first region 13 of the second conductivity type. This is a pin photodiode structure where the semiconductor substrate 11 is an n layer (or a p layer), the semiconductive crystal layer 12 is an i layer and the first region 13 is a p layer (or an n layer), and a photo-sensing region 18 is formed in the i layer. An electrode 14 of a second conductivity type is formed on the first region 13 on the surface of the semiconductor crystal layer 12, and the first region 13 inside the electrode 14 is covered with an anti-reflection film 16 while the semiconductive crystal layer 12 outside the electrode 14 is covered with a protection film (i.e. passivation film) 15.
When a reverse bias is applied to the semiconductor device thus constructed, a depletion layer is created in a pn junction area in the semiconductive crystal layer 12. Thus, an electric field is developed in the depletion layer and electrons and holes generated by a light applied to the photo-sensing region 18 are directed to the first conductivity type semiconductor substrate 11 and the second conductivity type region 13, respectively, and accelerated thereby. In this manner, a photocurrent is taken out and a light signal is detected.
In the structure shown in FIGS. 4 and 5, when the light is applied to the photo-sensing region 18, light generating carriers are captured by the depletion layer and a good response characteristic is offered. However, when the light is directed to the outside of the region 18, the generated carriers reach the pn junction while they are diffused by a density gradient and are taken out as a photocurrent. As a result, the response characteristic is adversely affected. FIG. 6 shows a response characteristic of the photodiode chip 1. Since the movement of the carriers by the diffusion is slow, a response waveform for a light pulse includes a tail at the end as shown in FIG. 6.
When such a photodiode chip 1 is used for the light communication, a light emitted from an optical fiber is condensed so that it is directed to the photo-sensing region 18. However, when a portion of light leaks out of the photo-sensing region 18, it leads to the reduction of the response speed of the photodiode chip 1 by the reason described above. In a high speed photodiode chip, the area of the photo-sensing region 18 is reduced to reduce a junction capacitance. As a result, a ratio of light directed to the outside of the photo-sensing region 18 increases and a diffused component which has a low response speed increases. This leads to the degradation of the response speed.
These conventional semiconductor light detecting devices have the following problems.
In a case in which a single mode fiber or a multimode fiber is used as the optical fiber 6, for, e.g., light of 1.3 .mu.m band, the core is about 10 .mu.m in diameter for the single mode fiber, and about 50 .mu.m in diameter for the multimode fiber. The light emitted from the end surface of the optical fiber disperses at an angle corresponding to a refractive index difference between the core and the cladding of the optical fiber 6.
The condenser lens 9 is essentially used to condense this dispersing signal light to the light detecting surface of the photodiode chip 1 as the light detecting means. In particular, in high-speed photodiodes of small light detecting diameters below 100 .mu.m, an aspherical lens or a selfoc lens with a better condensing property is often used. Thus expensive parts must be used.
Even in the case where this expensive lens is used, very high precision is required to align the lens 9 and the optical fiber 6. In particular, in fixing the condenser lens 9 to the part B in FIG. 1, to keep a dispersion of sensitivities of the pn junction as the light detecting region (first region 13) of the photodiode chip within .+-.0.5 dB, an allowance of the precision error must be about 15 .mu.m for the accurate alignment between the lens 9 and the optical fiber 6.
To fix the part, spot welding by a YAG laser (Yttrium Aluminum Carnet Laser) is most commonly used. But a positional displacement of the fixed part can take place, and the yield is as low as 70.about.80%. In addition it takes as much 20.about.30 minutes for one alignment.
What is especially a problem is a case where a condensed beam (signal light emitted from the end surface of the optical fiber) is radiated to parts other than the light detecting region of the photodiode chip (the arrow in FIG. 3 represents a divergence of a beam radiated from the core 6' of the optical fiber 6).
A part of the signal light absorbed in the light detecting region (first region 13) and its surroundings, (e.g., 3.about.5 .mu.m) contributes to the generation of a photoelectric current effectively at high speed because of an electric field applied to the pn junction, but because of the absence of an electric field a part of the signal light absorbed outside the light detecting region and its surroundings adversely generates a photoelectric current of very low response speed.
As a result of the latter case, in reproducing an analog signal light (photoelectric conversion), a resultant signal may be phase shifted with a high distortion level. A problem is that a reproduced signal waveform has noise. In reproducing a digital signal light (photoelectric conversion), a reproduced signal waveform has a distortion (in particular, a rectangular pulse has a dragging trailing edge of .mu.sec at the fall), and high-speed communication cannot be performed.
In these circumstances, to remove these problems the signal light emitted from the end surface of the optical fiber has be condensed to the light detecting region (pn junction) of the photodiode chip through an expensive and high-quality optical lens system.
Although the afore-mentioned semiconductor light detecting device is essential to optical communication, this particular semiconductor light detecting device has the above described problems and in addition, is expensive, which hinders the progressive prevalence of the optical communication system.